During construction of oil and gas wells, a rotary drill is typically used to bore through subterranean reservoirs of the earth to form a wellbore. As the rotary drill bores through the earth, a drilling fluid or mud is circulated through the wellbore. Drilling fluids are usually pumped from the surface into the wellbore through a drill string and transmitted to the drill bit. By continuously pumping the drilling fluid, the drilling fluid can be circulated out the bottom of the drill pipe and back up to the well surface through the annular space between the wall of the wellbore and the drill pipe.
Once the wellbore has been drilled, casing is lowered into the wellbore. A cement slurry is then pumped into the casing and a plug of fluid, such as drilling mud or water, is then pumped behind the cement slurry in order to force the cement up into the annulus between the exterior of the casing and the borehole. The cement slurry is then allowed to harden as a sheath. The cement sheath then holds the casing in place. The well is subsequently stimulated in order to enhance the recovery of oil or gas from the reservoir.
Maintaining zonal isolation for the lifetime of the well is critical. Leakage behind the casing reduces the cost-effectiveness of the well and may cause safety risks from pressure build-up. During well treatment operations, in particular completion and production phases of the well, variations in temperature and internal pressure of the wellbore pipe string may result in radial and longitudinal pipe expansion and/or contraction. This tends to place stress on the annular cement sheath existing between the outside surface of the pipe string and the inside formation surface or wall of the wellbore. Such stresses lead to cracking and/or disintegration of the cement sheath. Thus, failure of a cement sheath to provide zonal isolation may occur as a result of properties of the cement-casing interface, the cement-formation interface as well properties of the cement.
It has become increasingly important for service providers to provide to well operators cement mixes capable of withstanding specific downhole conditions well and specific operating conditions which the well is to be subjected.
Not only must the cement slurry containing such cement mixes exhibit a pumpable viscosity, acceptable fluid loss control, minimal settling of particles and the ability to set within a practical time, the cement mix and the properties of the cement slurry must be carefully selected in order to minimize or eliminate cracking of the cement when set as a cement sheath. As such, the cement mix and the slurry containing the mix must be tailored in order for the set cement to withstand those axial stresses, shear stresses and compressional stresses encountered under in-situ wellbore conditions. Further, the components of the cement mix and the cement slurry must be selected such that, when hardened, the set cement is not brittle since brittleness causes cracking of the sheath.
Knowledge of the bond strength of the cement sheath at the formation face and at the casing interface at in-situ downhole conditions would be an invaluable tool to ensure an optimal cement mix and job design are provided to the well operator. To date, however, neither a testing protocol nor an apparatus has been developed which is capable of measuring shear bond strength at both interfaces.
A testing method is needed for evaluating cement mixes and in particular the shear bond strength of a cement set from such mixes under conditions which simulate conditions found in a wellbore environment. The method needs to provide reliable and reproducible data at both the formation interface and at the casing interface. In particular, a testing protocol is desired for assessing bond strength at both the formation interface and at the casing interface which is based on a targeted subterranean formation to be cemented and bottom hole conditions of the well. Testing methods under these conditions will provide the requisite data for optimizing the properties of cementitious slurries for rendering suitable hardened cements at in-situ stress conditions.